proper fuel injector size after a 351W swap May 12, 2013, 08:26:40 PM Hi I was hoping that some one could help me determine the proper fuel injector size for my application.My Cougar has a 1970 block 351W the rotating assembly was balanced block was bored .030 over. Pistons are Keith Black flat tops http://www.summitracing.com/parts/uem-kb151030/overview/ cam is a summit SUM-4400 http://www.summitracing.com/parts/sum-4400stock mustang computer AL9 i believe stock mustang throttle body and mas air sensor. The injectors i am currently using are stock ford 25.3lb they were used in ford flex fuel vehicles.the car seems to run good but it is running rich and only gets about 9 MPG.I was not expecting phenomenal fuel mileage I have not even had it tuned yet but 9 seems a little low. is it because the injectors do not match the computer and MAF? should i just go back to my 19lb fuel injectors?Thank you any help would be appreciated. Quote Selected
proper fuel injector size after a 351W swap Reply #1 – May 12, 2013, 08:49:16 PM The MAF meter and injectors must match, it has no way of knowing the injectors are oversized.... On a 351 the air flow the meter is probably pegged so is signaling the PCM to increase pulse width to max... Guessing you could probably get away with a 24lb MAF with those inj... Quote Selected
proper fuel injector size after a 351W swap Reply #2 – May 13, 2013, 12:19:21 AM Yes, you need a calibrated MAF. Quote Selected
proper fuel injector size after a 351W swap Reply #3 – May 13, 2013, 12:27:56 AM yeah, different MAF, or put the 19's back in.but as mentioned, a half decent combo on a 351 will peg the MAF anyway Quote Selected
proper fuel injector size after a 351W swap Reply #4 – May 13, 2013, 08:54:26 AM The MAF does not give a shiznit about what size the injectors are, its the EEC that drives the injectors and we need to be clear on that. 19 lb/hr injectors will only support about 260 flywheel HP so that is waaay to small of an injector for a 351W but with a stock Fox MAF, stock Fox TB, and the cam listed the motor is going to make 302 power anyhow so they may be fine. If you want the motor to make power your best bet is to step up to a calibrated 80mm or larger MAF (not a big fan of these as they are a band aid) and some 36 lb/hr injectors (up to about 500 flywheel HP) and get a 70mm or 75mm TB on the car. This is all based on you having the cylinder heads and exhaust to move that much air and wanting to make power.Another thing you could look at doing is getting a Cobra EEC as it is setup for 24 lb/hr injectors which would agree with the ones you already have. This would most likely be the easiest thing to do and it would get the injector EEC mismatch fixed and without using a band aid like a calibrated MAF.Darren Quote Selected
proper fuel injector size after a 351W swap Reply #5 – May 13, 2013, 11:56:47 AM Quote from: Aerocoupe;415052The MAF does not give a shiznit about what size the injectors are, its the EEC that drives the injectors and we need to be clear on that. 19 lb/hr injectors will only support about 260 flywheel HP so that is waaay to small of an injector for a 351W but with a stock Fox MAF, stock Fox TB, and the cam listed the motor is going to make 302 power anyhow so they may be fine. If you want the motor to make power your best bet is to step up to a calibrated 80mm or larger MAF (not a big fan of these as they are a band aid) and some 36 lb/hr injectors (up to about 500 flywheel HP) and get a 70mm or 75mm TB on the car. This is all based on you having the cylinder heads and exhaust to move that much air and wanting to make power.Another thing you could look at doing is getting a Cobra EEC as it is setup for 24 lb/hr injectors which would agree with the ones you already have. This would most likely be the easiest thing to do and it would get the injector EEC mismatch fixed and without using a band aid like a calibrated MAF.DarrenA little conflicting info there eh? Trying to confuse the OP?(that I suspect is not really knowledgeable on the subject)True the EEC drives the injectors, but as you later suggest the MAF plays a huge role in what the EEC "thinks" is happening... I agree a large MAF CALIBRATED for the injectors used is what needed... BTW The Cobra EEC you recommend is likely a good choice but it's matching MAF is calibrated for that computer only, while most of the aftermarket MAF are for a std EEC... Quote Selected
proper fuel injector size after a 351W swap Reply #6 – May 13, 2013, 12:32:03 PM Don't worry guys I am not confused. I am fairly knowledgable on the subject I just wanted other people's opinions on it. My original idea when I was planing this build was that the larger injectors would compensate for me bringing in extra air because of the larger displacement. I can see now why that will not work. So my only options are to put my 19lb injectors back in or get a calibrated MAF. Will the 19lb work in this application? The heads are stock 1970 351w with a port job. Stock valve size and exhaust is shorty headers and 2.5" dual Quote Selected
proper fuel injector size after a 351W swap Reply #7 – May 13, 2013, 12:46:55 PM No I was not trying to confuse the OP I simply left out the part about having to have the Cobra MAF paired with the Cobra EEC so I do apologize for that. I would be very easy to just stat that rather than go about it the way you did but it is what it is.I ran across a very interesting thread on the Corral that sums up a lot of what we are talking about here, its a bit of a read but worth the info:http://forums.corral.net/forums/5-0-5-8-engine-tech/295186-1993-cobra-maf-sensor.htmlMy 306 with ported 69 351 heads maxed out the 24 lb/hr injectors so I would expect much of the same for your 351W.Darren Quote Selected
proper fuel injector size after a 351W swap Reply #8 – May 13, 2013, 01:06:57 PM If you are using a stock 55mm MAF, consider using it as a paper weight, it's going to be restrictive even with correct injectors...Unless you find a screamin' deal on the Cobra EEC & it's matching MAF something like this is probably your best bet... http://www.jegs.com/i/BBK+Performance+Products/129/8003/10002/-1 Quote Selected
proper fuel injector size after a 351W swap Reply #9 – May 13, 2013, 02:00:19 PM TurboCoupe50,Definitely a good example of what to do and that looks like a good piece to boot. This will most likely be the cheapest route to getting the car running right.Another possibility is to just suck it up and put a tune on the car which costs money. If you have access to a person that can dyno tune then I would suggest an SCT chip and tune. You can get an 80mm MAF from a 2001 GT Mustang and rewire it for the Fox harness, install some 36 lb/hr injectors, and get it tuned. I found the 2001 GT MAF for my coupe at http://www.car-parts.com for $50 shipped and the harness for $25 from http://www.promracing.com/parts-accessories/wiring-adapters.html. From there all you have to do is sell the injectors you have, find some 36 lb/hr or 42 lb/hr units, and then get the car tuned with a chip. All and all you will probably spend $750 and have a much better running car across the RPM range and make more HP out of the combination you have.Darren Quote Selected
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